Color imagers include color video cameras and color scanners for commercial printing. Color imagers transform color pictures into machine readable data. This is accomplished by dividing a color image into many small portions called pixels. The color imager separates light from each pixel into red, blue or green light. Numbers assigned to each pixel of the color image represent the red, blue, and green light. A fast, high resolution, accurate color imager would enhance the usefulness of computers and automate numerous tasks. For example, computers can print and display color images. However, the lack of fast, accurate, and high-resolution means for transferring color images into a computer limits the use of this capability.
In the early prior art, discrete optical components, such as beamsplitters and color filters, separate the color components of an image. Dichroic beamsplitters have been widely used due to their combined function as both beamsplitter and filter. Typically, color separation is achieved by placing two discrete dichroic beamsplitters in the optical pathway between the projection lens of the imager and its photosensors. The first dichroic beamsplitter reflects a first spectral band (e.g., green) to the first photosensor while transmitting the remaining spectral bands to the second dichroic beamsplitter. The second dichroic beamsplitter reflects a second band (e.g., red) to a second photosensor while transmitting the remaining spectral band (e.g., blue) to the third photosensor. The disadvantage of this approach is that the respective dichroic beamsplitters and photosensors must be precisely aligned; otherwise, the color components will not have the proper optical coincidence. The costly alignment process limits the use of this prior art color separator.
With the advent of low-cost, solid-state photodiode array photosensors, various attempts have been made to develop low cost color separation techniques for color scanners and video cameras.
Solid-state, photodiode arrays with integral color filters have been commercialized by Hitachi, Toshiba, Sony and RCA. These devices employ a two-dimensional array of photodiodes on a single silicon substrate. The array is coated with a gelatin layer, into which color dyes are selectively impregnated, using standard masking techniques. Each photodiode is, thus, given an integral color filter, e.g., red, green or blue, according to a color pattern which is repeated throughout the array. The same technology has been applied to one-dimensional photodiode array sensors for line scanners. The latter devices have been commercialized by Toshiba and Fairchild.
A prior art color imager using photodiode arrays is shown in FIG. 1. The single linear photodiode array 23 has individual organic dye filter impregnated over each photodiode in a repetitive red, blue and green pattern. Color separation, the breaking down of a color image into red, blue and green light, is achieved by focusing the light beam on the array, as shown in FIG. 1. One red, green and blue photodiode grouping 25 provides information to one color pixel. This prior art technique has several disadvantages. Since three photodiodes supply information to one pixel, the pixel resolution is reduced to one-third. For accurate color imaging, the luminance detail and chroma of a given color pixel from the original image must be resolved by three optically coincidental photosensor elements. However, the prior art photodiode arrays do not have color-coincidence. The red light is detected from one location, green from another, and blue from a third location. In addition, two-thirds of the light incident on each photodiode is lost by filter absorption (e.g., a red filter absorbs green and blue spectral bands). In order to increase the resolution, the array 23 must be lengthened or the photodiode area must be decreased. However, either of these approaches to increase the resolution will proportionately decrease scan speed. Also, the dye filters have less color band purity than dichroic filters. The prior art approach desaturates color sensitivity and is otherwise spectrally inaccurate.
Another prior art color imager using photodiode arrays has a rotating color wheel composed of colored filter segments. The lens focuses a line image of the original object on a linear photodiode array. The rotating color wheel filters the projected line image in a repeating color sequence, e.g., red, green, blue. The signal for each color component of a given line image is stored digitally until all three color components have been detected. The signals are then reordered in memory to assign three color values to each pixel in the line image.
The color wheel color separation technique has the advantages of utilizing the full resolution of the photodiode array as well as utilizing dichroic filters. However, it has several disadvantages. The scan speed is one-third of the integral sensor/filter scan speed, since only one of three colors is detected at a time. Also, further speed reduction results from transitions between filter segments during rotation of the wheel. When the color wheel and scan line are continuously driven, as opposed to synchronously "stepped", the effective resolution of the photodiode array is diminished in the scan direction by the movement of the scan line through the color cycle of the color wheel. Another disadvantage is the size of the color wheel which limits device extensibility. Page-width "contact" or "traversing head" type scanner embodiments become impossible or unwieldy. Further, this prior art device is burdened with a large moving mechanism and the control of this mechanism.
The Sharp Corporation of Japan has introduced a third prior art color separation technique for color document scanning. The Sharp scanner employs a single photodiode array with three sequentially-fired colored fluorescent lamps (e.g., red, green, blue), as the imaging light source. The sequence of signals obtained by the photodiode array is directly analogous to the color wheel color separator. That is, the input to the photodiode array is a sequential input of the red, green and blue components of a given original line image. Likewise, the photodiode signals for each color component are digitally stored and reordered in memory at the end of each color cycle.
Like the color wheel color separator, the tri-colored lamp approach provides imaging means that utilize the full resolution of the photodiode array. Several shortcomings, however, limit the speed and color integrity of the imager. In order to obtain correct color separation, the light output from each lamp should be extinguished before the firing of the next lamp in sequence; blended lamp output produces undersaturated color detection. Scanning speed, as a result, is limited by the persistence time of the phosphors utilized in each fluorescent lamp or the ability to dynamically subtract out the signal produced by the decaying light output of a previously fired lamp. Color integrity is further limited by the selection of phosphors having persistence values sufficiently low to meet commercial scan speed specifications. Typically, external absorption filtering of the lamps is required to obtain the desired spectral characteristics of each lamp output. As with the color wheel color separator, when the scan line is continously driven, as is desirable for scan speed, the effective resolution of the photodiode array is diminished in the scan direction by the movement of the scan line through the color cycle of the sequentially-fired lamps. The size and bulk of the optical system comprising the three lamps likewise restricts device extensibility toward "contact" or "traversing head" type scanner applications.